Direct Current (DC) power transmission systems are of interest to use in various situations, for instance when transferring electrical power over long distances.
It is known to transfer power between a DC power line and a three-phase alternating current (AC) power line with the three phases connected in series between the DC power line and ground or a negative DC bus.
In for instance U.S. Pat. No. 6,519,169 there are a number of power transfer modules connected in series between a DC power line and ground, where there is one such module per AC phase. Each module is made up of a two-level converter and an AC phase output is provided at the midpoints of two parallel branches of the module, where one branch includes switches with anti-parallel diodes and the other capacitors.
In recent years multilevel converters based on cascaded two-level cells have been used more frequently. These have the advantage of reducing or limiting the need for filtering equipment on the AC side.
In for instance WO 2007/028349 there is described a number of power transfer modules connected in series between a DC power line and ground. These modules are each made up two parallel branches, with each branch including a series connection of converter cells and with a transformer winding connected between the midpoints of the two branches.
The number of switching elements provided in this way is however fairly large and there would therefore be of interest to reduce this number.
One way of limiting the number of cells is described in the Session Paper “A New Hybrid Voltage-Sourced Converter for HVDC Power Transmission” by D. R. Trainer et al from the Cigré 2010 Sessions, Paris, France, 22-27 Aug. 2010. Here the power transfer modules are each made up of three branches, where two are made up of semiconductor switches and a third of two-level cells. Here a transformer winding is connected between the midpoints of the two semiconductor switch branches. WO 2010/088969 discloses the same type of arrangement.